Heretofore, the TAB tape and flexible printed circuit board generally have been manufactured by combining a polyimide film with a copper foil using a heat-curing adhesive or a thermoplastic adhesive. Most of the known adhesives are heat resistant only up to a temperature of 200° C. Hence, it cannot be employed in a manufacturing process in which a high temperature procedure such as soldering is involved. Moreover, these adhesives are not satisfactory in their electric performances.
Accordingly, it is desired to provide a copper-clad laminate comprising a polyimide film and a copper film and showing increased heat resistance as a whole. Moreover, there is an additional problem in that a sufficiently thin copper film cannot be placed on the polyimide film by the conventional manufacturing method employing a copper foil and an adhesive. As a result, it becomes difficult to form a fine copper pattern on the polyimide film by etching.
Recently, copper-clad laminates using no adhesive have been proposed. However, it is difficult to manufacture a copper-clad laminate having a high bonding strength (namely, a high peel strength) between the polyimide film and the copper foil, because the polyimide film has a surface of poor adhesiveness. Therefore, a number of trials for improving the adhesiveness of the polyimide film surface have been made. For instance, wet treatment processes such as desmear treatment and an alkali treatment have been known. The wet treatment process is, however, disadvantageous in that a sufficient washing procedure should be done and further a sufficient drying procedure should be done before a copper foil is placed on the wet treated surface. Also known are dry surface treatments such as a plasma discharge treatment and a corona discharge treatment. However, the known dry surface treatments cannot impart to the polyimide film a sufficiently high adhesiveness to the copper foil.
JP-A-6-124,978 discloses a copper-clad laminate which is prepared by coating a polyimide film (prepared from a biphenyltetracarboxylic dianhydride and p-phenylenediamine) with a polyimide (PMDA polyimide, prepared from pyromellitic dianhydride and 4,4′-diaminodiphenyl ether), heating the coated product to give an intermediate PMDA polyimide layer, and placing a metal-deposited layer (such as Cr-deposited layer) and a plated copper layer on the intermediate layer.
JP-A-7-197239 describes a polyimide substrate comprising an aromatic polyimide film, a sputtered subbing metal layer, and an extremely thin copper layer. The subbing metal layer comprises a metal selected from the group of Ni, Cr, Mo, W, V, Ti, and Mn. The copper layer has a thickness of not more than 50 nm.
JP-A-8-332697 describes a polyimide substrate comprising an aromatic polyimide film, a subbing metal alloy layer, and a copper layer. The metal alloy layer comprises an alloy of metals selected from the group of Ti, Co, Mo, and Ni. In the working examples, the metal alloy layer comprises a metal alloy in which Mo is less than 50 wt. %.
When the electro-conductive metal plated polyimide substrate is employed for manufacturing electronic parts such as TAB tape or flexible printed circuit board, the polyimide substrate is subjected to a procedure for washing a resist pattern out of the electro-conductive surface metal layer using an aqueous alkaline solution, and further to an etching procedure using a etching solution such as an alkaline ferric chloride solution or an alkaline cupric chloride solution. In the former washing procedure, the surface metal layer and the subbing metal layer should be resistant to erosion by the alkaline solution. In the latter etching procedure, not only the surface metal layer but also the subbing metal layer should be etched out as quickly as possible.